Lesson Heating and IAQ · Electric heat
Electric heat: diagnosis
Diagnosing electric heat is mostly electrical proof: call for heat, blower, voltage to sequencers/relays, and clamp amps on each stage. Temperature rise across the heater kit confirms heat is entering the air. Replace parts only after the circuit path is proven open or closed incorrectly.
Diagnostic order
- Thermostat call (W / E / Aux as applicable)
- Blower operation and filter
- Fuses / breakers / disconnect for heater kit
- Control voltage to sequencers or board outputs
- Line voltage at elements when stage commanded
- Amp draw vs expected (kW and voltage)
- Supply–return temperature rise
Expected amps (idea)
Amps ≈ watts ÷ volts (single-phase single element path; watch multi-leg kits). Nameplate is truth—calculate before condemning.
Sequencer failures
Classic: burnt sequencer leaves some stages dead while others work. Feel/amp each stage. Boards on newer furnaces need OEM status lights.
Open elements
Visual breaks or infinite ohms. Replace with correct kW kit; torque and ceramic hardware matter.
Heat pump strip issues
Outdoor unit may be fine while strips fail—or thermostat stuck in emergency. Measure both systems.
Temperature rise
OEM publishes allowable rise. Too high → low airflow. Too low with amps present → mixed air / sensor placement error, or undersized call.
Fuse and breaker patterns
One leg open on a multi-circuit kit yields partial kW and odd amp stories. Measure both legs. Time-delay fuses on heater kits are specific—substituting random fuses invites nuisance trips or unsafe circuits.
Wire and lug issues
Heat cycles loosen lugs; browned conductors at elements are a fire watch item. Torque to OEM values after service. Charred sequencers often follow high resistance connections upstream.
Temperature rise math check
If amps prove full kW but rise is tiny, air may be bypassing the heater or your probes sit in stratified flow. Move probes; verify duct heater is actually in the moving airstream.
Heat pump coexistence
When strips and the outdoor unit should share load, a miswired outdoor thermostat or control board can leave strips locked on. Diagnose by watching both systems during a call—not only the kit.
Practice on the job
Before you leave the topic in your head, force a slow walkthrough on the next real call:
- Name the component or condition you are standing in front of without looking at the ticket.
- Say out loud what “good” looks like for airflow, temperatures, and safety.
- Write one measurement you will take even if the customer wants a parts swap now.
- Tie the symptom to a cause you can prove—not a habit charge or a jumper.
- Recheck after the fix under the same ambient and load conditions you logged at the start.
That five-step habit turns book knowledge into field judgment. If you cannot explain the “why” to a helper on their first month, you do not own the lesson yet.
Tie-backs that prevent callbacks
Callbacks usually come from skipping the boring proofs: clean air paths, correct control sequence, solid liquid or safe vapor where the design expects it, and safeties that still work. When pressures look confusing, return to the physical story—heat in, heat out, metering, and compression—then re-measure. When a heating or IAQ complaint shows up, return to air, fuel/electric safety, and moisture before you upsell gadgets.
Keep OEM charts and nameplates above folklore. Typical numbers in training are compasses; the plate and the chart for today’s indoor/outdoor conditions are the map. Photograph settings you change so the next visit does not undo a careful repair.
Field case
Situation. 15 kW kit: only ~5 kW of heat. Amp one stage only. Sequencer contacts burned on stages 2–3.
How to think it.
- Diagnosis by stage amps finds partial failure fast.
- Replace sequencer; verify all stages; check for low airflow that overheated control.
- Full kW restored.
Takeaway: clamp meter > guessing.
In the field
Symptom
Little/no heat, tripped breakers, odors
Where to look
Stage amps, sequencers, limits, breakers
Likely causes
- Open element, sequencer, limit, wiring
What to measure
- V, A per stage, rise
What not to do
- Replace entire kit without stage testing
Checklist
- I follow call → blower → voltage → amps → rise
- I calculate expected amps from kW
- I test each stage separately
- I check heat-pump thermostat mode
- I fix airflow when rise is high